The invention relates to the continuous cooking of pulp.
To increase the productivity in existing continuous pulp digesters, different modifications have successively been made to the cooking technique. When the production capacity is increased in the digester, the flow of pulp through the digester increases, whereupon the dwell time is reduced at the cooking temperature, which temperature is necessary to maintain sufficient release of the lignin and dissolution of the pulp chips.
A natural step has been to take the impregnation step from the digester itself and arrange it in a pretreatment vessel prior to the actual digester. In this way it is possible to maintain the dwell time for the pulp chips in the digester and the cooking temperature despite the speed of the flow of pulp through the digester increasing.
As production increases, it is also desirable that the main extraction screen for used cooking liquor, called black liquor, is moved down in the digester, so that the length of the cooking zone is extended. The main extraction screen for consumed cooking liquor draws off warm and pressurized black liquor, and steam is generated by the pressure of the black liquor first being released in a flash tank. The black liquor is then taken for evaporation after which it is conveyed onwards to the recovery arrangement (recovery boiler).
This involves a conflict with the demands on achieving an effective wash zone at the bottom of the digester, which wash zone is intended on the one hand to wash out residual lignin but also to have the effect of lowering the temperature of the pulp.
Lowering the temperature to below about 100 degrees has been considered necessary so that the strength of the pulp is not reduced. If the pulp at a temperature of over 100xc2x0 C. is exposed to atmospheric pressure from the digester through a pressure-release delivery system, this results in blowing-off of heat, so-called flashing. If the temperature is substantially above 100xc2x0 C. (near the cooking temperature of xcx9c140-160xc2x0 C.) and the pulp pressure is released to 1 bar, this results in very powerful flashing on account of the cooking liquor""s conversion from liquid phase to steam phase, which greatly reduces the strength of the pulp.
To ensure a sufficient washing effect in the wash zone and a sufficiently low temperature in pulp blowing, the reduced length of the wash zone demands ever more powerful countercurrent flows of wash liquid in the wash zone. Particularly with increasing production in a given digester and with a constant dilution factor, the relative speed between liquor and chips increases, which results in increasing lift forces. This has a detrimental effect on the plug flow of pulp through the digester and tends to lift the whole pulp column in the digester, which effects both reduce the operability of the digester, with production shutdowns as a consequence.
U.S. Pat. No. 4,123,318 discloses a cooking system for pulp in which a specially adapted digester vessel is followed by two series-connected vessels for conventional countercurrent washing, i.e. the same type of washing as essentially always applies at the bottom of the digester.
EP-A-476,230 discloses a system in which a limited quantity of white liquor is added in the countercurrent zones during the extraction of consumed cooking liquor. Here, a heat exchanger is used for heating, in a recirculation loop above the bottom of the digester, the wash liquid delivered through the dilution nozzles. The pulp is fed to a diffuser which in normal circumstances is assumed to be an atmospheric diffuser, and where the wash liquid is assumed to be collected in a conventional manner from a downstream position in the fibre line. EP-A-476,230 states that the temperature in the countercurrent zones is increased to 140-175xc2x0 C., in sample tests 165xc2x0 C., and for a dwell time of 180 minutes. Here, full use has not been made of the fact that the dilution liquid/wash liquid added at the bottom of the digester will also already have this high temperature at the time of addition.
U.S. Pat. No. 5,066,362 discloses a digester and pressure diffuser system in which the pulp is taken from the bottom of the digester at temperatures of around 148-160xc2x0 C. (300-320xc2x0 F. in the text) and where the first stage of the pressure diffuser is provided with heated white liquor, expediently at the level of the blow temperature for the pulp. The aim here is to obtain an extended delignification of kraft pulp.
In a variant in said U.S. Pat. No. 5,066,362, only wash liquid from a subsequent drum wash is used, and at temperatures of around 74xc2x0 C. (166 F. in the text) of the wash filtrate from the drum wash. Here, a countercurrent wash is established in a conventional manner at the bottom of the digester, where filtrate from the pressure diffuser is fed as wash liquid at the bottom of the digester and extracted via a screen arranged at a distance from the bottom of the digester. Thus, the wash liquid moves counter to the descending movement of the wood chips. The cooking liquor extracted from the screen is then led to a flash tank.
This document also includes extraction of some of the pressure diffuser filtrate to the flash tank, which sub-quantity only represents the excess which is not needed for the necessary amount of wash liquid in the wash zone. This system does not fully use the establishment of a co-current flow of cooking liquor and wood chips down through the whole digester, which impairs the operability particularly if production is to be increased as the flow speed of the wood chips has to be increased.
SE-C-501,848 (=EP 670,924; U.S. Pat. No. 5,919,337) has proposed a system in which a higher temperature can be maintained across substantially the whole of the digester, in so-called ITC cooking. This document has discussed the advantage of having the same pressure in the pulp flow""s transfer to a so-called pressurized diffuser, which was at the bottom of the digester. The wash filtrate from the pressure diffuser is recirculated in full back to the bottom of the digester and has, upon recirculation, a temperature of 100xc2x0 C., maximum 110xc2x0 C., resulting in a wash zone/temperature-reducing zone at the bottom of the digester. Cooking liquor/wash liquid is extracted in a screen immediately above the bottom of the digester and is recirculated to this level via a heat exchanger so that the cooking temperature can be maintained over the lowest placed screen. The pulp issuing from the digester has a temperature of 105-115xc2x0 C. Using the innovative solution of a pressure diffuser directly after the digester, which pressure diffuser is capable of working at digester pressure levels of 10-20 bar, there is no flashing directly after the digester. This eliminates the problems of blowing to atmospheric pressure from 105-115xc2x0 C., which would cause an explosion-like disintegration of the pulp fibres.
In connection with special digesters for handling branch wood chips/sawmill chips, special problems arise when a very high degree of packing is obtained, which normally makes effective extraction of cooking liquor from the whole pulp column impossible using screens in the wall of the digester. The branch wood chips and sawmill chips represent raw materials with most of their content in fine fractions well below the normally well-defined wood chips for cooking.
Normal wood chips for cooking are obtained using chippers which give wood chips with lengths of about 20-25 mm.
The sawmill chip fraction is often defined as the fine fraction, or the material which passes through a sieve with round holes of about 3 mm.
The branch wood chip fraction is often defined as the intermediate fraction, or the material which passes through sieves with holes exceeding 3 mm but below 8 mm (where sawmill chips have already been sieved out). Thus, wood chips normally contain long slivers which can be allowed to pass through such a sieve.
The accepted part of the wood chips often has a content where the main part, more than 60%, often around 75-80%, consists of chips which pass through sieves with holes larger than 7 mm, but do not pass through sieves with 8 mm slits. Well-defined wood chips have most of their content within this range, and very small quantities of fine fractions.
When cooking branch wood chips/sawmill chips, use has previously been made of continuous digesters which are fed with wood chips and cooking liquid at the top, after which the pulp column is allowed to descend through the digester without extraction. In this particular context, pressure diffusers have been used as a wash connected directly after the cooking, and where the pressure diffuser has maintained the pressure from the digester. Here, the filtrate from the pressure diffuser has either been taken in its entirety for recovery and at high temperature, typically up to 150xc2x0 C., or limited flows of about 25% have been returned to the digester outlet as dilution liquid, but then at temperatures of about 120xc2x0 C. for the dilution liquid.
The invention relates to an improved process in which it is possible to increase production capacity, primarily in existing digesters for cooking of wood chips, but also in new installations, while maintaining a high degree of operability in a cooking process with a digester with an extended cooking zone without powerful countercurrent flows of cooking liquor or wash liquid in the digester and particularly at the bottom of the digester. By this means it is possible to obtain a stable and continuous column movement of the pulp volume down towards the bottom of the digester.
At the bottom of the digester there is a very high degree of packing, inter alia because of the fact that the chips are softened during the chemical dissolution process and the pressure from above chip column increases. If a countercurrent wash zone is to be located at the bottom of the digester, a bottom screen with a high extraction capacity has to be used in order to be able to establish an effective countercurrent which can give a washing out effect.
To be able to establish a net countercurrent flow overall, very large amounts of free liquid must be circulated, as it is necessary to compensate for the liquid which is bound in the chips and which is taken from the digester together with the chips. This is particularly noticeable in overloaded digesters where the speed of the chip column movement is very high.
If instead it is possible to accept that only the net flow of liquid at the bottom of the digester de facto moves downwards, a certain limited counterflow of free liquid can be allowed to move upwards. The wash effect from such limited countercurrent flows is however very limited. The problem of operability arises particularly in those cases where there is a powerful countercurrent flow of free liquid. By ensuring that the net flow of liquid at the bottom of the digester does not move upwards, in accordance with the invention, the operability of the digester is increased.
The definition of co-current zone thus signifies all zones where at least the net flow of the liquid has a movement which coincides with the descending movement of the chips. This means that in these zones the free liquid can still move upwards, but then with relatively limited amounts of liquid, which can be drawn off with a bottom screen even in the case of overloaded digesters.
In the most preferred embodiment of the invention, however, the digester is operated in such a way that the free liquid at the bottom of the digester also moves downwards.
Another object is to make it possible to minimize the extraction flows to be drawn off from the digester and then conveyed onwards to recovery (via blow tank evaporation and finally the recovery boiler). Major problems exist today in running overloaded digesters in particular, as relatively large extraction flows of consumed cooking liquor (black liquor) are to be obtained at typically just one single screen position far down in the digester, very near the lowermost wash zone screen. For reasons of flow technology, it is also often impossible in practice to draw off consumed cooking liquor at a speed higher than 0.03 m/s from the compressed pulp column, which means that it is impossible to draw off all the consumed cooking liquor from the entire cross section of the pulp column. At the same time it is difficult to draw off large amounts of consumed cooking liquor without disturbing/affecting the chip column movement.
Another object is to obtain a cooking zone which de facto uses the whole digester, and also to some extent continues after the digester, which means that the digester capacity can be increased even more, by increasing the flow speed of the pulp through the digester.
Another object is to move the main extraction of cooking liquor from the digester to an apparatus downstream of the digester, which apparatus is better suited to draw off the cooking liquor. In this way, the main extraction of consumed cooking liquor away from the process takes place not from an extraction screen arranged in the periphery of the digester, where the extraction is to draw off consumed cooking liquor from a pulp column with a diameter in the range of 5-12 meters.
A further object is to maintain a high temperature in the pulp, achieving improved heating economy, avoiding the heat losses which unavoidably occur in blowing of cooking liquor, and reheating of cooking liquors by means of indirect heat exchangers.
To avoid said disintegration of the digested pulp, which reduces its strength, the invention proposes that a pressurized wash apparatus be connected directly downstream of the digester and that the pulp be fed to this wash apparatus without any real decrease in pressure. A marked drop in pressure takes place only after the pressurized wash where the temperature of the pulp and its alkali content have dropped to such a level that the fall in pressure consequently has little or no negative effect on the quality of the pulp. Such a wash apparatus can advantageously consist of a pressure diffuser, also affording the advantage of being able to use the hot and pressurized extract from this pressure diffuser as dilution liquid at the bottom of the digester. This substantially improves the heating economy and at the same time results in reduced pump energy and reduces the need for cumbersome large heat exchangers.